letters to nature
178 NATURE | VOL 406 | 13 JULY 2000 | www.nature.com
Most samples give 28–33% O2 yield during laser fluorination. We found by analysing
NBS127 (NIST sulphate standard), seawater sulphate, and a low-d18O in-house
standard that the raw d18O values are consistently 9.4 lower than the reported ones, owing
to the incomplete O2 generation. The correction factor is also verified by more than a
dozen natural sulphate samples—that have been analysed using both the laser-fluorina-
tion method and the graphite reduction/CO2-fluorination method
27—that normally
reach a 85% to 100% O2 yield in our laboratory. This comparison also verifies that the
incomplete O2-generation using a CO2-laser does not deviate from the slope of 0.52. Thus,
the reported d17O value is increased by 4.89‰. d34S was analysed using the SF6 method
28.
In this report, all D17O values are calculated on the basis of raw d17O and d18O values. We
use the linear equation to calculate D17O because all the raw d17O and d18O values are close
to the vicinity of the origin and the range of d17O and d18O values among different samples
are small.
Received 3 January; accepted 25 May 2000.
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Acknowledgements
We thank T. Jackson for technical assistance, J. Cannia for sand samples from Scotts Bluff,
Nebraska, J. Alt for marine ferric oxide samples, J. Savarino for helpful discussions, and
NASA and NSF for support.
Correspondence and requests for materials should be addressed to H. B.
(e-mail: hbao@chem.ucsd.edu).
.................................................................
Determining multiple length
scales in rocks
Yi-Qiao Song, Seungoh Ryu & Pabitra N. Sen
Schlumberger-Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877,
USA
................. ......................... ......................... ......................... ......................... .........................
Carbonate reservoirs in the Middle East are believed to contain
about half of the world’s oil1. The processes of sedimentation and
diagenesis produce in carbonate rocks microporous grains and a
wide range of pore sizes, resulting in a complex spatial distribu-
tion of pores and pore connectivity2. This heterogeneity makes it
difficult to determine by conventional techniques the character-
istic pore-length scales, which control fluid transport properties.
Here we present a bulk-measurement technique that is non-
destructive and capable of extracting multiple length scales
from carbonate rocks. The technique uses nuclear magnetic
resonance to exploit the spatially varying magnetic field inside
the pore space itself—a ‘fingerprint’ of the pore structure. We
found three primary length scales (1–100 mm) in the Middle-East
carbonate rocks and determined that the pores are well connected
and spatially mixed. Such information is critical for reliably
estimating the amount of capillary-bound water in the rock,
which is important for efficient oil production. This method
might also be used to complement other techniques3–5 for the
study of shaly sand reservoirs and compartmentalization in cells
and tissues.
Nuclear magnetic resonance (NMR) is often used to investigate
porous media by applying external magnetic field gradients in a
fashion analogous to X-ray diffraction in determining the size of
cells6 and pores7–9. In fact, when a sample is placed in a uniform
magnetic field, a spatially varying field and its gradients appear
naturally inside the pore space as a result of the magnetic suscep-
tibility contrast between the host solid material and the pore-filling
fluid10. This field is called the internal field, Bi, and it can be large
enough in natural materials such as rocks11–14 to interfere with the
100 µm
25 µm
c
grain grain
grain
grain
b
a
Figure 1 Thin-section micrographs of the pore space in Thamama carbonate rocks and
an illustration of the internal magnetic field. a, b, The blue regions are pores filled with
blue epoxy before sectioning. Pores of size a few mm to about 100 mm are clearly visible.
The smaller pores show up as different level of shades owing to limited resolution. It is
crucial to determine these length scales in understanding the transport properties.
c, Diagram of the typical pore space, internal magnetic field B i and diffusion. The blue
lines illustrate the constant B iz contours (the component along the external field) reflecting
the local pore geometry. The magnetization decay due to diffusion in B i is used to
characterize the pore sizes.
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